| The δ13C of photosynthate provides an annually resolved record of intrinsic water-use
efficiency, which is the ratio of photosynthesis to stomatal conductance. This has made it
possible to correlate δ13C with rising temperatures and atmospheric CO2 concentrations. But
the attractiveness of this approach should be tempered by awareness of its limitations,
including confounding due to height effects, nitrogen deposition, smearing due to reserves,
post-photosynthetic fractionations, and especially mesophyll conductance. The literature has
dealt with these issues as everything from quibbles to fatal flaws. This talk will argue that
the problems are minor if δ13C is used as an index of change over time, but they
are more severe for quantitative scaling from physiological processes to global
atmospheric composition. Progress on the quantitative scaling front will be highlighted. |